This invention relates to an improvement in monostable multivibrators.
A conventional monostable multivibrator is constructed, for example, as shown in FIG. 1, by cascade-connecting a gate circuit (for example, an OR circuit) 1, a driver circuit 3 comprising a saturating transistor switch circuit including a transistor 2, a differentiating circuit 6 consisting of a capacitor 4 and a resistor 5 and a voltage comparison circuit (for example, an inverter) 7 where the output of voltage comparison circuit 7 is fed back to one input of gate circuit 1. The input signal is supplied to the other input of gate circuit 1 and a pulse output synchronized with the input signal is obtained from the output terminal of voltage comparison circuit 7.
When this circuit is converted to an integrated circuit, because driver circuit 3 is a saturating transistor switch circuit, high frequency, AC current flows to the substrate of the integrated circuit causing anomalous phenomena and unstable operation of the monostable multivibrator. In order to avoid this anomalous phenomena, it is necessary to make all circuits a non-saturating type, where a non-saturating transistor switch circuit would be used in the driver circuit of the differentiating circuit. The non-saturating transistor switch circuit may, for example, comprise an emitter-coupled differential amplifier such as shown in FIG. 2, in which the emitters of transistors 8 and 9 are connected in common. The common connection point is grounded through a constant current source 10, and the collector of transistor 8 is connected through a load resistor 12 to power supply +B where the collector of transistor 9 is directly connected to the power supply. In addition, although the output of the driver circuit is shown as connected to the collector of transistor 8, it may also be taken from transistor 9.
When the above-mentioned differential amplifier shown in FIG. 2 is used as a driver circuit 3' instead of the driver circuit 3 shown in FIG. 1, the reference level (In this specification, the low level side of a positive pulse and the high level side of a negative pulse are denoted as reference levels) of the output of the driver circuit 3' is dependent upon the voltage V.sub.B of the power supply +B causing no problem. However, the peak level (In this specification, the high level side of the positive pulse and the low level side of the negative pulse are denoted as peak levels) is dependent upon the current of constant current source 10 and fluctuates according to the noise current of constant current source 10. Therefore, a shortcoming exists in that the peak level of the output of driver circuit 3' fluctuates and a change in the amplitude of the driving pulse that drives differentiating circuit 6 appears as a fluctuation in the output pulse width of the monostable multivibrator. Thus, for example, when the monostable multivibrator using the driver circuit 3' shown in FIG. 2 is used in a pulse count wave detector of an FM receiver, there is a shortcoming in that a change in the output pulse width appears as a change in the DC voltage of the wave detector output and deteriorates the SN ratio of the wave detector output.